It’s true that in general you should not use a dimmer designed for light to control a variable speed motor like a fan.
However, and this only applies to the UK, there are now some universal dimmer micros coming out which can be used to control some, but not all, variable speed fans provided the current draw is low enough.
The Qubino is one possibility and also the new aeotec nano ( but not the older Aeotec dimmer).
Again, this would only apply in the UK, don’t try it in the US.
I would talk to vesternet tech-support about any specific fan model that you are considering. They are generally very helpful.
And again, this applies to UK devices only. Not US devices. The US versions are generally only spec’d for Lighting Control.